Voltage conversion and charging from low bipolar input voltage

ABSTRACT

A circuit includes a transformer configured with a primary winding and a secondary winding that are driven from a voltage supplied by a thermoelectric generator (TEG). The circuit includes a bipolar startup stage (BSS) coupled to the transformer to generate an intermediate voltage. The BSS includes a first transistor device coupled in series with the primary winding of the transformer to form an oscillator circuit with an inductance of the secondary winding when the voltage supplied by the TEG is positive. A second transistor device coupled to the secondary winding of the transformer enables the oscillator circuit to oscillate when the voltage supplied by the TEG is negative. After startup, a flyback converter stage can be enabled from the intermediate voltage to generate a boosted regulated output voltage.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication 61/767,120 filed on Feb. 20, 2013, and entitled CIRCUIT ANDMETHOD FOR A COMPLETELY ELECTRONIC STARTUP FROM EXTREMELY LOW POSITIVEOR NEGATIVE INPUT VOLTAGES. This application also claims the benefit ofU.S. Provisional Patent Application 61/767,129 filed on Feb. 20, 2013,and entitled CIRCUIT AND METHOD FOR FAST AND EFFICIENT CHARGING FROMEXTREMELY LOW POSITIVE OR NEGATIVE INPUT VOLTAGES. The entirety of eachof the above-identified applications is incorporated by referenceherein.

TECHNICAL FIELD

This disclosure relates to a voltage regulator circuit, and moreparticularly to a boost converter circuit for thermoelectric harvestinghaving self-starting capability from a de-energized state.

BACKGROUND

Advances in low-power biomedical and industrial sensor designs have madeenergy harvesting an attractive alternative to batteries for poweringimplanted or otherwise hard-to-reach sensors. Such constraints on thelocation of these sensors also precludes the use of solar or vibrationenergy harvesters as a source of energy. Thermal energy harvesting is asuitable alternative due to the presence of reasonable thermal gradientsin industrial settings, or between the human body and the environment,for example. Thermoelectric generators (TEGs) can be employed forthermal energy harvesting and are capable of powering downstreamelectronic circuits from the TEG.

Bulk-mode TEGs typically produce 20-30 mV for every one Kelvin oftemperature difference across them, with an output impedance as low astwo ohms. Harvesting energy using these devices usually implies workingoff an output voltage as low as 50 mV in the worst case, where thetemperature difference across the device is about two Kelvin. Boostconverters are typically used to convert these voltages sufficiently towhere CMOS circuits can subsequently be powered, for example. Startingup these converters proves to be a significant challenge since mostswitches in the converters have threshold voltages far exceeding theoutput of the TEG. In one example, this problem has been addressed byincorporating a battery to operate the switches during startup.Alternatively, a motion-activated mechanical switch can be employed.These approaches can significantly increase the cost and complexity ofcircuit integration and implementation, however.

In addition to the low output voltage magnitude produced by TEG's, thepolarity of the induced voltage output depends on the direction of heatflow, which can vary across different applications or within the sameapplication. Two separate transformer-based oscillators can be used insome applications for both low-voltage startup and steady-stateoperation—one each for each polarity. A transformer-based oscillator forstartup and a transformer-based boost converter can be used in otherapplications, leading to higher efficiency but only with the capabilityto support a single polarity. Using multiple transformers also canincrease the cost and complexity of a given circuit implementation.

SUMMARY

This disclosure relates to a boost converter circuit for thermoelectricharvesting having self-starting capability from a de-energized state.

In one example, a circuit includes a transformer configured with aprimary winding and a secondary winding that are driven from a voltagesupplied by a thermoelectric generator (TEG). The circuit includes abipolar startup stage (BSS) operatively coupled to the transformer togenerate an intermediate voltage greater than the voltage supplied bythe TEG. The BSS includes a first transistor device operatively coupledin series with the primary winding of the transformer to form anoscillator circuit with an inductance of the secondary winding when thevoltage supplied by the TEG is positive. A second transistor deviceoperatively coupled to the secondary winding of the transformer enablesthe oscillator circuit to oscillate when the voltage supplied by the TEGis negative.

In another example, a circuit includes a transformer configured with aprimary winding and a secondary winding that are driven from a voltagesupplied by a thermoelectric generator (TEG). A flyback converter stage(FCS) is operatively coupled to the transformer to generate an outputvoltage greater than the voltage supplied by the TEG. A first transistordevice is operatively coupled in series with the primary winding of thetransformer to excite the primary winding and to generate an AC outputon the secondary winding when the output voltage is below apredetermined threshold. A second transistor device can be operativelycoupled in parallel with the first transistor device. The secondtransistor device is operatively coupled in series with the primarywinding of the transformer to excite the primary winding and to generatean AC output on the secondary winding when the output voltage is abovethe predetermined threshold.

In another example, a circuit includes a transformer configured with aprimary winding and a secondary winding that are driven from a voltagesupplied by a thermoelectric generator (TEG). A bipolar startup stage(BSS) is operatively coupled to the transformer to generate anintermediate voltage greater than the voltage supplied by the TEG. TheBSS includes a first transistor device operatively coupled in serieswith the primary winding of the transformer to form an oscillatorcircuit with an inductance of the secondary winding when the voltagesupplied by the TEG is positive. A second transistor device isoperatively coupled in series with the secondary winding of thetransformer to enable the oscillator circuit to oscillate when thevoltage supplied by the TEG is negative. The circuit includes a flybackconverter stage (FCS) operatively coupled to the transformer to generatean output voltage greater than the intermediate voltage supplied by theBSS. The FCS includes a third transistor device operatively coupled inseries with the primary winding of the transformer to excite the primarywinding and to generate an AC output on the secondary winding when theoutput voltage is below a predetermined threshold. The FCS also includesa fourth transistor device operatively coupled in parallel with thethird transistor device, the fourth transistor device operativelycoupled in series with the primary winding of the transformer to excitethe primary winding and to generate an AC output on the secondarywinding when the output voltage is above the predetermined threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of schematic block diagram of a bipolarstartup and boost converter circuit to regulate a voltage supplied by athermoelectric generator.

FIG. 2 illustrates an example phase diagram that illustrates the variousphases that can be executed by the bipolar startup and boost convertercircuit of FIG. 1.

FIG. 3 illustrates an example of a bipolar startup stage that can beemployed for the bipolar startup and boost converter circuit depicted inFIG. 1.

FIG. 4 illustrates an example of a flyback converter stage that can beemployed for the bipolar startup and boost converter circuit depicted inFIG. 1.

FIG. 5 illustrates an example of a combined bipolar startup and flybackconverter circuit that can be employed in the bipolar startup and boostconverter circuit depicted in FIG. 1.

FIG. 6 illustrates a sense and monitor circuit to detect power levels ina flyback converter stage and to cycle the flyback converter stage ifthe detected power level falls below a predetermined threshold.

DETAILED DESCRIPTION

This disclosure relates to a boost converter circuit for thermoelectricharvesting having self-starting capability from a de-energized state. Acircuit topology is provided that employs a single transformer in anoscillator circuit that operates as a self-starting oscillator whenstarting up from small positive or negative input voltages supplied by athermoelectric generator (TEG). After startup, the transformeroscillator functionality is disabled and the same transformer is thenmultiplexed through different operating modes (e.g., via controlcircuits) until a steady state output voltage has been attained. As usedherein, the term multiplexed refers to using control circuits to operatethe transformer in one mode such as oscillation mode during startup andthen automatically reconfigure the transformer into another mode such asboost converter mode (also referred to as flyback mode) after startup.Thus, the single transformer can be multiplexed between differentcircuit configurations as the converter proceeds through startuptransients until steady-state operation has been achieved whileoperating from an input of either polarity.

During the initial phase of startup, the converter operates as anoscillator satisfying the oscillation criteria for bipolar inputs andcharges a low-capacitance intermediate node. After a voltage thresholdhas been attained at the intermediate node, a lower-efficiencyintermediate flyback stage can then be employed to speed up the startupprocess and begin charging a high-capacitance output node. Asub-regulator (e.g., a regulator that regulates internal voltages withinthe boost converter circuit) can be activated when the voltage on thehigh-capacitance node has attained an intermediate voltage threshold tofurther facilitate the charging process. A high-efficiency main flybackstage can then be activated upon charging the high-capacitance outputnode to a suitable level. Both flyback stages and the oscillator sharethe same multiplexed transformer which mitigates circuit costs. Bipolaroperation of the flyback converter can be achieved by flux commutationin the transformer core by resonating the self-inductance of the primarywith its parasitic capacitance.

FIG. 1 illustrates an example of schematic block diagram of a bipolarstartup and boost converter circuit 100 to regulate a voltage suppliedby a thermoelectric generator 104. As used herein, the term circuit caninclude a collection of active and/or passive elements that perform acircuit function, such as a voltage regulator or the like. The termcircuit can also include an integrated circuit where all the circuitelements are fabricated on a common substrate, for example. The circuit100 includes a transformer 110 configured with a primary winding and asecondary winding that are driven from a voltage VSUP supplied by thethermoelectric generator (TEG) 104. The voltage VSUP from the TEG 104can be a positive or negative polarity depending on the direction ofheat flow that is sensed by the TEG. During startup of the circuit 100,a bipolar startup stage (BSS) 114 is engaged with the transformer 110via a cold start control block 120 to generate an intermediate voltageV-INT that is greater than the voltage supplied by the TEG 104. The BSS114 includes a bipolar oscillator circuit 124 (e.g., FET and/or junctiontransistors) that operate in conjunction with the transformer 110 duringthe startup phase of the circuit 100 to form an oscillator circuit tocharge an intermediate capacitance node shown as C-INT 128. Output fromC-INT 128 provides the voltage V-INT.

As will be illustrated and described below with respect to FIG. 3, afirst transistor device in the bipolar oscillator circuit 124 can becoupled in series with the primary winding of the transformer 110 toform a self-starting oscillator circuit with an inductance of thesecondary winding when the voltage supplied by the TEG 104 is positive.The self-starting oscillator circuit formed by the bipolar oscillatorcircuit 124 and transformer 110 can be considered a variation of aMeissner oscillator or Armstrong oscillator, for example. If aconventional Meissner configuration were employed with the transformer110, where only a single transistor were employed in the oscillatorcircuit, then oscillations would stop when VSUP from the TEG 104 changesto negative. Thus, a second transistor in the bipolar oscillator circuit124 can be coupled to the secondary winding of the transformer to enablethe oscillator circuit to oscillate when the voltage supplied by the TEGis negative. The second transistor operates to provide a 180 degreephase shift to maintain oscillation when VSUP from the TEG 104 isnegative as will be described below. Other transistors in the bipolaroscillator circuit 124 can be employed to enable and disable operationsof the BSS 114 as will be illustrated and describe below with respect toFIG. 3.

The cold start control block (CSCB) 120 monitors V-INT until apredetermined threshold is attained (e.g., 0.6 V). The CSCB 120 caninclude discrete monitors (e.g., comparators) and output drivers tomonitor the V-INT threshold and enable/disable the bipolar oscillatorcircuit 124. This can also include a controller that executesinstructions to operate the CSCB 120. When the V-INT threshold has beenexceeded, the CSCB 120 disables the bipolar oscillator circuit 124 andactivates a ring oscillator 134 that is also powered from V-INT. In oneexample, the ring oscillator 134 can be an oscillator circuit composedof an odd number of NOT gates whose output oscillates between twovoltage levels, representing true and false. The NOT gates, orinverters, can be attached in a chain where the output of the lastinverter is fed back into the first.

When the ring oscillator 134 is activated, an intermediate boosttransistor 140 is cycled (e.g., turned off and on) in a flybackconverter stage 144 in response to the ring oscillator. The flybackconverter stage 144 operates with the transformer 110 to boost thevoltage on a larger capacitance C-STORE 150 which supplies voltage tooutput V-STORE. Bipolar operation of the flyback converter 144 can beachieved by flux commutation in the transformer core by resonating theself-inductance of the primary with its parasitic capacitance. Duringinitial charging of C-STORE 150, the intermediate boost transistor 140is activated since it operates at a lower threshold voltage that can besupported by the ring oscillator 134. As the ring oscillator 134oscillates, the voltage V-INT can drop below the V-INT threshold. Thus,the ring oscillator 134 can be disabled and the bipolar oscillatorcircuit 124 reactivated to boost the voltage on C-INT 128 andcorrespondingly drive V-INT back above the V-INT threshold. During thistime of transition between charging states, C-STORE 150 can be decoupledfrom the output V-STORE and thus hold its charge from the previouscharging cycle applied by the intermediate boost transistor 140 andtransformer 110.

As V-STORE rises above an intermediate threshold, portions of a maincontrol block (MCB) 160 monitoring V-STORE can activate a sub-regulator170 which regulates a voltage between V-STORE and V-INT. By activating,the sub-regulator 170, the charging time of C-STORE 150 can be reduced.The sub-regulator 170 can be a linear regulator, in one example,although other regulator types can be utilized as the sub-regulator.After V-STORE has risen above a V-STORE threshold set by the MCB 160,the ring oscillator 134, intermediate transistor 140, and bipolarstartup stage 114 can be disabled. The MCB 160 can then cycle (e.g.,turn-off and turn-on) a main boost transistor 180 that is coupled inparallel with the intermediate boost transistor 140 and is now employedto operate the flyback converter stage 144 with the transformer 110.

Like the intermediate boost transistor 140, the main boost transistor180 is coupled in series with the primary winding of the transformer 110to excite (e.g., via current flow through) the primary winding and togenerate an AC output on the secondary winding when the regulated outputvoltage V-STORE is above the predetermined threshold set for V-STORE.The main boost transistor 180 operates at a higher turn-on thresholdthan the intermediate boost transistor 140. The higher thresholdoperation promotes greater efficiency in the flyback converter stage 144after V-STORE has risen sufficiently (e.g., as determined bythresholds). After V-STORE has risen above the V-STORE threshold, thesub-regulator 170 can be deactivated by the MCB 160. As will beillustrated and described below with respect to FIG. 4, a single outputrectifier (e.g., diode) can be employed at the output of the secondaryof the transformer 110 to generate a DC charging current for C-STORE 150in the flyback converter stage 144. After charging C-STORE 150, theregulated output voltage V-STORE can then be employed to power otherelectronic circuits (e.g., batteries, regulators, controllers, monitors,sensors, and so forth).

FIG. 2 illustrates an example phase diagram 200 that illustrates thevarious phases that can be implemented by the bipolar startup and boostconverter circuit described above with respect to FIG. 1. The differentphases of the diagram 200 are shown with respect to time which flowshorizontally from left to right on the diagram. Four time periods, T1,T2, T3, and T4 describe the time periods the respective phases of thediagram 200, are active. At T1, during cold start (e.g., in the absenceof a battery), the bipolar oscillator charges CINT. When V-INT crosses acold start power-on threshold (e.g., V-INT at about 0.6V), the flybackconverter is then operated in intermediate-boost mode during time periodT2 to charge C-STORE until V-INT falls below a threshold (e.g., ahysteretic threshold). The bipolar oscillator then takes over againduring T2 to charge C-INT back to the power on threshold level. Duringthis period of T2, there is minimal leakage current from C-STORE sincethe main sensing and control circuits have not been activated, and henceC-STORE holds most of its stored charge.

The charging rate of C-STORE can be further increased by connectingC-STORE and C-INT through a sub-regulator when V-STORE exceeds V-INT byan intermediate threshold voltage beginning at time period T3. Thecharging action of C-STORE speeds up at a higher rate when thesub-regulator is activated. When the capacitor C-STORE is charged to athreshold of about 1.8 V, the main control block 160 activates the mainoscillator (e.g., main boost transistor in flyback oscillator) and othercontrol circuits are activated while the main primary side transistor isutilized. The control circuits for the main flyback converter canoperate at about 2 kHz, for example, although other frequencies arepossible. Upon reaching steady state operation, the bipolar oscillator124, the cold start control block 120 and the ring oscillator 134driving the low-efficiency flyback converter (e.g., intermediate boosttransistor) are disabled using PMOS gating switches, for example, toreduce loss in the converter.

FIG. 3 illustrates an example of a bipolar startup circuit 300 that canbe employed in the bipolar startup stage 114 and boost converter circuit100 depicted in FIG. 1. The circuit 300 includes a transformer 310configured with a primary winding 314 and a secondary winding 318 thatare driven from a voltage (VSUP) supplied by a thermoelectric generator(TEG) (not shown). A bipolar startup stage (BSS) 320 is operativelycoupled to the transformer 310 to generate an intermediate voltage VINTgreater than the voltage VSUP supplied by the TEG. A first transistordevice 324 is operatively coupled in series with the primary winding 314of the transformer 310 to form an oscillator circuit with an inductanceof the secondary winding 318 when the voltage VSUP supplied by the TEGis positive. A second transistor device 328 is operatively coupled viaits gate terminal to the secondary winding 318 of the transformer 310 toenable the oscillator circuit to oscillate when the voltage VSUPsupplied by the TEG is negative. In one example, the first transistor324 and the second transistor 328 are native metal oxide semiconductordevices (e.g., native NMOS) to facilitate efficient operations of theoscillator circuit since native devices are processed to provide verylow turn-on thresholds (e.g., 0.01 volts to 0.1 volts). In electronics,a native transistor (or natural transistor) is a variety of the MOSfield-effect transistor that is intermediate between enhancement anddepletion modes. Most common is the native n-channel MOS transistor.

The BSS 320 includes a first depletion MOS (e.g., PMOS) device 334operatively coupled in series with the first transistor 324. A seconddepletion MOS (e.g., PMOS) device 338 is operatively coupled in serieswith the second transistor 328. The first and second depletion MOSdevices 334 and 338 are employed to enable the BSS 320 during a startupphase and to disable the BSS after the startup phase. The first MOSdevice 334 can be configured as a controllable switch that is on whenenabled during the startup phase and when disabled after the startupphase off (e.g., turned on and off by CSCB described above). The secondMOS device 338 can be controlled as a linear resistor (e.g., operate MOSdevice in linear region) during the startup phase and can be disabledafter the startup phase. As shown, the BSS 320 also includes a couplingcapacitor 344 to couple an AC output voltage from the secondary winding318 of the transformer 310 to an AC output (AC OUT) of the BSS 320. Arectifier circuit 350 (e.g., half-bridge or full-bridge diode circuit)converts the AC output from the BSS 320 to the DC output voltage VINTfor the BSS 320. An intermediate storage capacitor C-INT 360 filters theDC output voltage of the BSS 320 for a subsequent converter stage, suchas the flyback converter stage described above (e.g., converter stage144 of FIG. 1).

As shown, the circuit 300 can employ a single transformer to achievebipolar startup capability. The start-up can be autonomous (e.g.,self-starting) in the absence of a battery or other external powersource other than thermoelectric generator that supplies VSUP to thetransformer 310. The circuit 300 can achieve bipolar startup by addingan amplifier branch via transistor 328 to the feedback loop from thesecondary winding 318 that performs the opposite of transistor 324 inthe primary branch. Thus, the transistor 328 acts as a source followerfor positive inputs and as a common-source amplifier for negative inputssupplied from VSUP. Hence, when the input VSUP is positive, noadditional phase is added to the loop and the oscillations happen, suchas in a Meissner oscillator configuration. When the input VSUP isnegative, the path provided by transistor 328 adds 180° phase to theloop which completes the phase requirement for oscillation of the BSS320. In some examples, where turn-off and turn-on capability are notneeded, the BSS 320 can operate without transistor devices 334 and 338which would be replaced by a conductor across the drain and sourcelocations of transistor device 334 and replaced by a resistor whichwould connect the drain and source locations of transistor 338. The sametransformer 310 can also be employed to operate the flyback converterwhich is described below with respect to FIG. 5.

FIG. 4 illustrates an example of a flyback converter circuit 400 thatcan be employed in the bipolar startup and boost converter circuitdepicted in FIG. 1. A transformer 410 is configured with a primarywinding 414 and a secondary winding 418 that are driven from a voltage(VSUP) supplied by a thermoelectric generator (TEG) (not shown). Aflyback converter stage (FCS) 420 is operatively coupled to thetransformer 410 to generate an output voltage (V-STORE) greater than thevoltage supplied by the TEG. The FCS includes a first transistor device424 having a first turn-on threshold is operatively coupled in serieswith the primary winding 414 of the transformer 410. The firsttransistor 424 is employed to excite the primary winding 414 and togenerate an AC output on the secondary winding 418 when the regulatedoutput voltage V-STORE is below a predetermined threshold. A secondtransistor device 428 having a second turn-on threshold is operativelycoupled in parallel with the first transistor 424. The second transistor428 is also operatively coupled in series with the primary winding 414of the transformer 410 to excite the primary winding and to generate anAC output (AC-OUT) on the secondary winding 418 when the regulatedoutput voltage V-STORE is above the predetermined threshold. Thetransistor 424 is driven by SW-INT supplied by the ring oscillatordescribed above. The transistor 428 is driven from an output SW-MAINsupplied by the main control block described above.

The first turn-on threshold of the first transistor 424 is lower thanthe second turn-on threshold of the second transistor 428. The lowerthreshold turn-on for the first transistor 424 supports a low outputvoltage from the ring oscillator during startup. However, the lowerthreshold is not as efficient when the output V-STORE approaches aregulation threshold set for V-STORE. Thus, after the regulationthreshold has been achieved for V-STORE, the second transistor 428having the higher turn-on threshold can be employed to operate the FCS420. The higher threshold for the second transistor 428 facilitateshigher efficiency in the FCS 420.

As shown, the FCS 420 also includes a rectifier 430 (e.g., diode) toconvert the AC output from the secondary winding of the transformer to aDC output V-STORE of the FCS 420. A storage capacitor C-STORE 434filters the DC output of the FCS 420 and stores electrical energysupplied from the rectifier. As will be illustrated and described belowwith respect to FIG. 6, the FCS 420 can also include a sense circuitoperatively coupled to the secondary winding 418 of the transformer 410to detect a power level of the FCS 420. A power control circuit canmonitor the sense circuit and periodically cycle the FCS 420 if thedetected power level drops below a predetermined threshold.

A signal diagram at 450 illustrates how flux commutation operates withinthe flyback converter stage 420. Transformers employed for typicalthermal harvesting applications generally have large turns ratios andthus large primary winding capacitances (e.g., for a turns ratio of1:100, a primary winding capacitance of 84 nF). Due to this largeparasitic capacitance on the primary 414, the node V-PRI on the primarycannot rise a diode drop above V-STORE, even if a diode was insertedbetween nodes V-PRI and V-STORE. Thus, when the switch 424 or 428 turnsoff under a positive supply VSUP, the flux in the transformer corecommutates (or changes polarity) as a result of the resonance betweenthe primary winding inductance and its associated parasitic capacitance.This flux commutation can be used to turn on diode D2 at 430 andtransfer the energy stored in the transformer core to V-STORE underpositive supply VSUP, which would otherwise have required separatediodes for a positive and negative VSUP.

FIG. 5 illustrates an example of a combined bipolar startup and flybackconverter circuit 500 that can be employed in the bipolar startup andboost converter circuit depicted in FIG. 1. A transformer 510 isconfigured with a primary winding 514 and a secondary winding 518 thatare driven from a voltage (VSUP) supplied by a thermoelectric generator(TEG) (not shown). A bipolar startup stage (BSS) 520 is operativelycoupled to the transformer 510 to generate an intermediate voltage VINTgreater than the voltage supplied by the TEG. A first transistor 524 isoperatively coupled in series with the primary winding 514 of thetransformer 510 to form an oscillator circuit with an inductance of thesecondary winding 518 when the voltage VSUP supplied by the TEG ispositive. A second transistor 528 is operatively coupled in series withthe secondary winding 518 of the transformer 510 to enable theoscillator circuit to oscillate when the voltage VSUP supplied by theTEG is negative.

A flyback converter stage (FCS) 530 can be can be operatively coupled tothe transformer 510 to generate a regulated output voltage (V-STORE)that is greater than the intermediate voltage V-INT supplied by the BSS520. A third transistor 534 (e.g., intermediate transistor) having afirst turn-on threshold is operatively coupled in series with theprimary winding 514 of the transformer 510 to excite the primary windingand to induce an AC output (AC OUT) on the secondary winding when theregulated output voltage V-STORE is below a predetermined threshold. Afourth transistor 538 (e.g., main transistor) having a second turn-onthreshold is operatively coupled in parallel with the third transistor534. The fourth transistor 538 is operatively coupled in series with theprimary winding 514 of the transformer 510 to excite the primary windingand to generate an AC output on the secondary winding when the regulatedoutput voltage V-STORE is above the predetermined threshold. As noted inthe example above, the first turn-on threshold of the third transistor534 is lower than the second turn-on threshold of the fourth transistor538. The lower threshold of transistor 534 supports the ring oscillatoron startup whereas the higher threshold of transistor 538 promoteshigher efficiency when the FCS 530 attains steady state.

As shown, the BSS 520 can include a first depletion MOS 544 operativelycoupled in series with the first transistor 524. A second depletion MOS548 can be operatively coupled in series with the second transistor 524.The first and second MOS devices are employed to enable the BSS during astartup phase and to disable the BSS after the startup phase (e.g., viaEn signals supplied by cold start control block). The first MOS 544 isconfigured as a controllable switch that is on when enabled during thestartup phase and off when disabled after the startup phase. The secondMOS 548 is controlled as a linear resistor during the startup phase andis disabled after the startup phase. The BSS 520 can also include anisolation switch 550 to isolate V-INT when the FCS 530 is operating.Similar to the circuit described above with respect to FIG. 3, the BSS520 can also include a coupling capacitor 554, a rectifier circuit 560,and intermediate storage capacitor C-INT 564.

An isolation switch 570 can be provided in the FCS 530 to isolateC-STORE 574 when the BSS is operating during start-up and before theV-INT threshold has been attained. Control signal En1 is employed tocontrol transistors 544, 548, and 550 in the BSS 520. Control signal En2and its compliment/En2 are employed to control isolation switch 570. Thecontrol signals En1 and En2 can be generated by a cold start controlblock 120 previously described with respect to FIG. 1. A rectifier 580in the FCS 530 rectifies AC OUT from the secondary 518 which is suppliedas a DC output to C-STORE 574 via switch 570.

As shown above with respect to FIG. 1, a cold start control block (CSCB)(not shown) can monitor the intermediate output voltage V-INT of the BSS520 during the startup phase and enables the first MOS 544 and thesecond MOS 548 when the intermediate output voltage V-INT is below apredetermined threshold. The CSCB disables the first MOS 544 and thesecond MOS 548 when the intermediate output voltage V-INT is above thepredetermined threshold. A ring oscillator not shown) can be enabled bythe CSCB to operate the third transistor 534 during an intermediateconverter phase. A main control block (MCB) (not shown) cycles thefourth transistor 538 after the intermediate converter phase. Asub-regulator (not shown) can be activated by the MCB when the regulatedoutput voltage V-STORE of the FCS attains an intermediate voltagethreshold and is deactivated by the MCB when the regulated outputvoltage of the FCS attains a storage threshold (e.g., 1.8 V). As will beillustrated and described below with respect to FIG. 6, a sense circuit(not shown) can be operatively coupled to the secondary winding 418 ofthe transformer 510 to detect a power level of the FCS 530. A powercontrol circuit (not shown) monitors the sense circuit to periodicallycycle the FCS if the detected power level drops below a predeterminedthreshold.

FIG. 6 illustrates a sense and monitor circuit 600 to detect powerlevels in a flyback converter stage and to cycle the flyback converterstage if the detected power level falls below a predetermined threshold.A circuit portion of the FCS stage described above with respect to FIG.5 is shown at 610. Voltage can be monitored from the secondary of theFCS stage 610 via a sense circuit 620. Output from the sense circuit 620can be sent to a power control circuit to periodically cycle the FCSstage depending on the detected power levels of the sense circuit 620.In some instances, the thermoelectric energy harvester's output can dropto arbitrarily low power levels depending on the operating conditions.Below a given threshold, for example, the net positive energy harvestedby the converter becomes negative, where more energy is consumed inswitching and powering the control and sensing circuitry than isharvested. In this instance, it can be beneficial to shut down theconverter until more favorable conditions arise (e.g., higher thermalgradients increase the voltage output of the thermal generator).

The main flyback converter can include an on/off detection circuit thatperiodically checks the status via the sense circuit 620. To detectwhether the converter is on, the maximum value of the voltage at theanode of diode DS can be stored on a capacitor Cp in the sense circuit620. When the converter is on, the voltage stored on the capacitor Cp isa diode drop above V-STORE. A resistor, Rleak, can be placed in parallelto Cp to leak away the stored energy when the diode DS does not turn on.This voltage can be compared against V-STORE using a latch 640 (e.g., astrong-ARM latch), which can be implemented within the power controlcircuit 630 having a configured input offset that is approximately equalto the value of a diode DS drop.

The latch 640 in the power control circuit 630 signals the converter tobe on only if VSUP is above V-STORE by a value greater than the inputoffset. When the converter is on, the output of the latch can be checkedat the 2 kHz system clock frequency, for example, in the power controlcircuit 630. As a further example, when the converter shuts off, in theabsence of adequate power to harvest, typically, only the controlcircuits that run on the 2 kHz system clock with quiescent current(e.g., 330 nA) remain on. The converter can be turned on for a pluralityof (e.g., about 16) cycles every two seconds, for example, by the powercontrol circuit 630 to determine if the thermoelectric harvester issupplying energy. If it is determined as such, the converter can be lefton.

What have been described above are examples. It is, of course, notpossible to describe every conceivable combination of components ormethodologies, but one of ordinary skill in the art will recognize thatmany further combinations and permutations are possible. Accordingly,the disclosure is intended to embrace all such alterations,modifications, and variations that fall within the scope of thisapplication, including the appended claims. As used herein, the term“includes” means includes but not limited to, the term “including” meansincluding but not limited to. The term “based on” means based at leastin part on. Additionally, where the disclosure or claims recite “a,”“an,” “a first,” or “another” element, or the equivalent thereof, itshould be interpreted to include one or more than one such element,neither requiring nor excluding two or more such elements.

What is claimed is:
 1. A circuit comprising: a single transformerconfigured with a primary winding and a single secondary windingconfigured to receive a voltage supplied by a thermoelectric generator(TEG); and a single bipolar startup stage (BSS) operatively coupled tothe single transformer to generate an intermediate voltage greater thanthe voltage supplied by the TEG, the BSS comprising: a first transistordevice operatively coupled in series with the primary winding of thesingle transformer and configured to receive a first signal to form anoscillator circuit with an inductance of the single secondary windingwhen the voltage supplied by the TEG is positive; and a secondtransistor device operatively coupled to the single secondary winding ofthe single transformer and configured to receive a second signal to formthe oscillator circuit which oscillates when the voltage supplied by theTEG is negative, wherein the second transistor device is configured as asource follower to the first transistor device for positive inputs fromthe TEG and the second transistor device is configured as a commonsource amplifier to the first transistor device for negative inputs fromthe TEG.
 2. The circuit of claim 1, wherein the first transistor deviceand the second transistor device comprise metal oxide semiconductors. 3.The circuit of claim 1, further comprising a first depletion metal oxidesemiconductor (MOS) operatively coupled in series with the firsttransistor device and a second depletion metal oxide semiconductor (MOS)operatively coupled in series with the second transistor device, thefirst and second metal oxide semiconductors to enable the BSS during astartup phase and to disable the BSS after the startup phase.
 4. Thecircuit of claim 3, wherein the first MOS is configured as acontrollable switch that is on when enabled during the startup phase andoff when disabled after the startup phase, and the second MOS iscontrolled as a linear resistor during the startup phase and is disabledafter the startup phase.
 5. The circuit of claim 1, further comprising:a single coupling capacitor to couple an AC output voltage from thesingle secondary winding of the single transformer to an AC output ofthe BSS; a single rectifier circuit to convert the AC output from theBSS to the intermediate voltage for the BSS; and a single intermediatestorage capacitor to filter the intermediate voltage of the BSS for asubsequent converter stage.
 6. The circuit of claim 5, furthercomprising a flyback converter stage (FCS) operatively coupled to thesingle transformer to generate an output voltage greater than theintermediate voltage supplied by the BSS, the FCS comprising: a thirdtransistor device having a first turn-on threshold operatively coupledin series with the primary winding of the single transformer and drivenby a third signal to excite the primary winding and to generate an ACoutput on the single secondary winding when the output voltage is belowa predetermined threshold; and a fourth transistor device having asecond turn-on threshold operatively coupled in parallel with the thirdtransistor device, the fourth transistor device operatively coupled inseries with the primary winding of the single transformer and driven bya fourth signal to excite the primary winding and to generate an ACoutput on the single secondary winding when the output voltage is abovethe predetermined threshold.
 7. The circuit of claim 6, wherein thefirst turn-on threshold of the third transistor device is lower than thesecond turn-on threshold of the fourth transistor device.
 8. A circuitcomprising: a transformer configured with a primary winding and a singlesecondary winding that are driven from a voltage supplied by athermoelectric generator (TEG); a bipolar startup stage (BSS)operatively coupled to the transformer to generate an intermediatevoltage greater than the voltage supplied by the TEG, the BSScomprising: a first transistor device operatively coupled in series withthe primary winding of the transformer and configured to receive a firstsignal to form an oscillator circuit with an inductance of the singlesecondary winding when the voltage supplied by the TEG is positive; anda second transistor device operatively coupled to the single secondarywinding of the transformer and configured to receive a second signal toadd 180° phase to the loop to form the oscillator circuit configured tooscillate when the voltage supplied by the TEG is negative; a flybackconverter stage (FCS) operatively coupled to the transformer to generatean output voltage greater than the intermediate voltage supplied by theBSS, the FCS comprising: a third transistor device operatively coupledin series with the primary winding of the transformer to excite theprimary winding and to generate an AC output on the single secondarywinding when the output voltage is below a predetermined threshold; anda fourth transistor device operatively coupled in parallel with thethird transistor device, the fourth transistor device operativelycoupled in series with the primary winding of the transformer to excitethe primary winding and to generate an AC output on the single secondarywinding when the output voltage is above the predetermined threshold. 9.The circuit of claim 8, wherein the third transistor device has aturn-on threshold that is lower than a corresponding turn-on thresholdof the fourth transistor device.
 10. The circuit of claim 8, furthercomprising a first depletion metal oxide semiconductor (MOS) operativelycoupled in series with the first transistor device and a seconddepletion metal oxide semiconductor (MOS) operatively coupled in serieswith the second transistor device, the first and second metal oxidesemiconductors to enable the BSS during a startup phase and to disablethe BSS after the startup phase.
 11. The circuit of claim 10, whereinthe first MOS is configured as a controllable switch that is on whenenabled during the startup phase and off when disabled after the startupphase, and the second MOS is controlled as a linear resistor during thestartup phase and is disabled after the startup phase.
 12. The circuitof claim 10, further comprising a cold start control block (CSCB) tomonitor the intermediate output voltage of the BSS during the startupphase and to enable the first MOS and the second MOS when theintermediate output voltage is below a predetermined threshold and todisable the first MOS and the second MOS when the intermediate outputvoltage is above the predetermined threshold.
 13. The circuit of claim12, further comprising a ring oscillator that is enabled by the CSCB tooperate the third transistor device during an intermediate converterphase.
 14. The circuit of claim 13, further comprising a main controlblock (MCB) to cycle the fourth transistor device after the intermediateconverter phase.
 15. The circuit of claim 14, further comprising asub-regulator that is activated by the MCB when the output voltage ofthe FCS attains an intermediate voltage threshold and is deactivated bythe MCB when the output voltage of the FCS attains a storage threshold.16. The circuit of claim 8, further comprising: a sense circuitoperatively coupled to the single secondary winding of the transformerto detect a power level of the FCS; and a power control circuit tomonitor the sense circuit and to periodically cycle the FCS if thedetected power level drops below a predetermined threshold.